QXcbNativeIntegration: Add query for compositing enabled

Task-number: QTBUG-41195
Change-Id: I4f37c82f6757283ed58b38c7fd47849fb4810bce
Reviewed-by: Błażej Szczygieł <spaz16@wp.pl>
Reviewed-by: Shawn Rutledge <shawn.rutledge@theqtcompany.com>

src/plugins/platforms/xcb/qxcbnativeinterface.cpp

@@ -85,7 +85,8 @@ static int resourceType(const QByteArray &key)
         QByteArrayLiteral("rootwindow"),
         QByteArrayLiteral("subpixeltype"), QByteArrayLiteral("antialiasingEnabled"),
         QByteArrayLiteral("nofonthinting"),
-        QByteArrayLiteral("atspibus")
+        QByteArrayLiteral("atspibus"),
+        QByteArrayLiteral("compositingenabled")
     };
     const QByteArray *end = names + sizeof(names) / sizeof(names[0]);
     const QByteArray *result = std::find(names, end, key);
@@ -246,6 +247,10 @@ void *QXcbNativeInterface::nativeResourceForScreen(const QByteArray &resourceStr
     case RootWindow:
         result = reinterpret_cast<void *>(xcbScreen->root());
         break;
+    case CompositingEnabled:
+        if (QXcbVirtualDesktop *vd = xcbScreen->virtualDesktop())
+            result = vd->compositingActive() ? this : Q_NULLPTR;
+        break;
     default:
         break;
     }

src/plugins/platforms/xcb/qxcbnativeinterface.h

@@ -74,7 +74,8 @@ public:
         ScreenSubpixelType,
         ScreenAntialiasingEnabled,
         NoFontHinting,
-        AtspiBus
+        AtspiBus,
+        CompositingEnabled
     };
 
     QXcbNativeInterface();